1. Field of the Invention
The present invention relates to an optical data recording medium, such as an optical disk or the like and a manufacturing method for this medium, and more specifically, to an optical data recording medium suitably used in a WO (write once) type optical disk and the like provided, for example, with a recording layer composed of an organic dye type heat mode recording material and a manufacturing method for this medium.
2. Prior Art Discussion
At present, a CD (Compact Disk) for reproducing music has been widely used. Since, however, this CD is used for reproduction only and does not have a so-called DRAW (Direct Read After Write) function, a user cannot make a recording on such CDs or edit them. There is, thus, a strong desire for a CD having this DRAW function. Further, in an optical disk having applications other than a CD, a less expensive optical disk including the DRAW function is desired.
A metal material such as Te, for an abration type medium or a metal material, such as In, for a so-called phase changing type medium effecting dislocation from crystal to non-crystal is considered to be useful as a void type optical recording material having the DRAW function Since, however, a recording layer composed of these metal type recording materials is formed using a thin film formation technique such as a vapor-deposition method, a sputtering method or the like, there remains a problem in mass-productivity and cost.
Thus, a pit type (heat mode WO type) organic dye type recording material which can be formed into a film by a spin coating method is considered more viable with respect to productivity.
An optical disk, in particular, an optical disk using an organic dye as a recording material must be provided with a protective layer for protecting a recording layer, and this protective layer is preferably formed by a spin coating method in consideration of manufacturing costs, mass productivity and the like. Then, in general, a CD or the like is provided with a protective layer composed of an acrylate resin or the like formed by a spin coating method.
When, however, a UV curing type acrylate resin or the like, which is widely used and advantageous in material costs, productivity and characteristics, is coated on a recording layer composed of the above organic dye type recording material, there are possibilities that the recording layer will be damaged and, at worst, when a UV curing type resin is coated, the dye will melt and be completely removed.
In addition, it has been proposed to provide the kind of optical data recording medium which comprises a transparent substrate composed of a polymethyl methacrylate resin or glass having a thin film of a dye, such as squalilium, thiopyrylium or the like, formed thereon and pits formed on the thin film by irradiation of a laser beam. (D. J. Gravesteijn et al., SPIE 420. 327, 1983).
In addition to the above, there are many proposals for the material, such as a cyanine dye (Japanese Patent Kokai (Laid-Open) No. 58-125246, Japanese Patent Kokai No. 59-85791), a naphtoquinone dye (Japanese Patent Kokai No. 58-224793), an azulenium dye (Japanese Patent Kokai No. 59-129954), a naphthalocyanin dye (Japanese Patent Kokai No. 61-25886) and the like. Further, another proposal uses a single state oxygen quencher together with the cyanine dye to improve an optical stability (Japanese Patent Kokai No. 59-67092). All of these proposals intend to record data by defining pits on a recording layer composed of a single dye layer.
Further, there have been proposed a reflecting layer formed on a plastic film in a flattened layer, a recording layer composed of a mixture of dye and polymer are formed on the reflecting layer and recording pits are defined by irradiation of a laser beam (James W, Wheeler et al., SPIE 420. 39, 1983), that a recording layer composed a mixture of dye and styrene oligomer is formed (A. Kuroiwa et al., Jap. J. Appl. Phys. 22. 340, 1983) and the like, and on all of which data are recorded by formation of pits.
When a recording layer is composed of a single dye of these various organic dyes, the dye remains on the bottom portions the pits defined by irradiation of a laser beam and the laser beam irradiated at the portion where the dye remains is reflected. Therefore, there is no difference between reflection factors at a flat portion and that at a pit portion, and thus a sufficient modulation factor cannot be obtained. Further, since the pits have a rim portion of the edges thereof shaped to a gentle and obscure configuration, this recording medium has a problem in that a signal output does not rise sharply.
Further, as described above, the recording medium provided with a recording layer composed of a mixture of the dye and polymer or the mixture of the dye and styrene oligomer has lower light absorption and light reflection than those of the recording layers composed of a single dye, and thus has a lower modulation factor. Further, not only this recording medium requires a larger power when a laser beam is irradiated thereon but also the configuration of the pits thereof is not clearly defined like the recording medium using the single dye so that the medium has a problem in recording reliability.
Further, with the above pit type optical recording medium, when a recording layer is formed on a transparent substrate and a laser beam is irradiated thereon, the recording layer is heat-melted at the portion where the beam strikes and the thickness of the recording layer made thinner or the substrate exposed at this portion.
In reproduction, a signal can be read out because a reflection factor of the portion is lowered. Since, however, the difference between the reflection factors at a non-recorded portion and that at a recorded portion is small, a sufficient modulation factor cannot be obtained and reproduction errors are caused.
In addition, there is also a known method that a metal film is expanded by vapor pressure produced by partially heating an organic film by the irradiation of a laser beam thereby to form bubbles for recording data.
Since, however, this method reproduces a signal by making use of reflection effected on a surface of a metal film and this metal film is formed by a thin film formation technique such as a vapor-deposition method, a sputtering method or the like, there remains a problem in mass-productivity and cost.
Conventionally, there exists a recording system wherein pits are defined by heat-melting and expanding a recording layer by irradiation of a laser beam. When a conventional dye and this irradiation are applied to a recording layer, declining of the viscosity according to the duration causes a restriction for good recording. As a means for solving this problem, there is an example wherein a material which is effectively subjected to a thermal effect by using a thermally unstable dye is employed. This thermally unstable material can improve sensitivity in recording, whereas it is degraded by reading light in reproduction. In addition, even if a thermally unstable material is used, a pit is only shaped to have an enlarged inner surface like a conical earthenware put and it is impossible to define good pits having clear pot edges.
Further, the dye is usually unstable under high light conditions, in particular under sunlight, and thus this recording medium has a drawback that the dye is observed to be degraded.
Thus low recording sensitivity and low resistance to light are problems of the prior art.
A process for manufacturing an optical recording medium comprising a heat-deformable optical recording layer and a protective layer of a transparent thermal and mechanical barrier layer are layered on the support thereof is disclosed in U.S. Pat. No. 4,340,655.
In this process, the protective layer is formed on the recording layer by applying a water solution of a water-soluble polymer to the recording layer to be dried.
However, the above prior art has the following problems:
(1) A water solution of the water-soluble polymer, in the case of production of an optical data recording medium by application of the solution to a recording layer to form a protective layer composed of a water soluble polymer, has the problems in that the recording material in the recording layer is dispersed or dissolved in water in the solution, or in that the material having wettability can not be used since the water of the solution permeates into the recording layer;
(2) Although a recording material difficult to dissolve in water is used, it is impossible to produce a medium composed of the recording layer and the protective layer, unless the recording material of the recording layer is wettable by the application solution;
(3) It is further necessary that a material having surface activity is used as a water soluble polymer component in the case that the recording material is water repellent.